Glorious mantras of Lord shiva

mantra mean "powerful word". mantras are the ones that have when chanted produce great effects. These are chanted repeatedly and that is called japa. japa is a key part of Hindu prayer.
mantras are very rich in their meaning. While doing japa one can meditate on the mantra - its meaning. As the mind dwell more and more into that, the mantra conditions the mind and takes up to the higher states and forms the path to the great liberation - eternal bliss !
What makes mantras so special as compared to the normal words ? mantras are not human composed. One may wonder how can that be possible. Especially given that there are sages associated with the mantras ! The point to be noted is that these sages are not composers of these mantras, as we normaly compose the sentences; they are not the inventors, but they are the discoverers of the mantra. They get to know the mantras in a state in which these words do not emanate from their thoughts, but they are just passive audience to it. Those who go deep in meditation and realize God may be able to get a feel of this situation.
To be such a discoverer, even though they are just passive hearers, needs great amount of qualification. Only the perfect one can unchangedly reproduce the mantra heard. The only one that is absolutely perfect is God. All other discoverers reproduce that mantra only as pure as their closeness to perfection.
veda samhitAs are full of mantras and hence have been preserved for ages in their pure form by utilizing the various techniques like patha, krama, jaTa, gaNa pATas, that ensure that the chanter clearly gets the correct letters and even the correct level of sound for each letter (svara). The chanters are advised to chant the mantras only after getting the right pronunciation of it, so that the mantras are presered against deterioration with time. There would be gurus who initiate the disciple in a mantra. guru ensures that the disciple got the mantra right, so that the person can chant independently as well as initiate others in that mantra. Ensuring this preservation vedas were passed only through the tradition of guru and disciples and was never written down till very recent past. (It is really amazing to note that without being written down the vedas have been preserved in pure form across the land by these techniques. Though the texts arem across the land by these techniques. Though the texts are freely available now for anybody to read, it would be important to ensure that these mantras are properly learnt and then chanted. This way the treasure that as been preserved so carefully over multiple milleniums do not deteriorate due to indifference.)
It is to be noted that many of the hymns of thirumuRai(1) are known to have great powers of mantras that are practiced even today.
While there are plenty of mantras available, there are a few that are chanted with high esteem by the shaivas. Definitely those are highly powerful ones that can lead the chanter on the great path to mukti (liberation). praNava, paNJchAkashra, gAyatri to name a few. For shaivites the Holy Five Syllables (paNJchAkshara) with or without combined with the praNava is the ultimate mantra.

Maasil Veenaiyum Maalai Madhiyamum
Veesu thendralum veengila veynilum
Moosu vandarai poygayum ponradhee
Eesan endhai inayadi neezhalee.

Translation:
The shelter under the parallel feet of the God, my boss, is like the nice melody of Veena (Musical Instrument), the early evening moon, pleasant breeze, the Spring, a pond where bees are in making sound.

Finally the king decided to throw Thirunaanukkarasar into the sea tieing him to a big stone. He was taken in a boat to the deeper portion of the sea with the body tied to a stone. He was thrown in the sea. He was unruffled. That time he recited with great love and confidence on Lord Shiva this hymn. To the surprise of all others the stone became the boat and Thirunaavukkarasar got down in a village near Thiruppadhiripuliyoor (Kadaloor).

Sotrunai veedhiyan soodhi vaanavan
Potrunai thirundhadi porundha kai thozha
Katrunai pootiyoor kadalir paaichchinum
Natrunai aavadhu NAMASHIVAYAvee

Translation:
The reciter of the Vedas, the flame like God, if we pray with folded hands to his gold like glittering feet, even if we were tied to a big stone and thrown in the sea there is a savior - that is the NAMASHIVAYA. ( The Holy five letter)

Thirunaavukkarasar wanted the bad statement that he once went to Jainism to be suppressed by identifying him as a Shivanadiyaar (Devotee). For that he wanted the holy symbols of the Lord, namely the Holy Bull and Thrisuula, to be marked on his body. For that he sang this song to the Lord of Thiruth thuungaanai maadam. Immediately Siva ganas came and put those marks on his shoulders and disappeared.